1. Field of the Invention
The present invention relates to a method for the recovery of fluorine in solid state from aqueous aluminum nitrate solutions containing fluorine ions. The method is particularly suitable for use in recovering uranium-free aluminum fluoride after conversion of UF.sub.6 to UO.sub.2.
2. Description of the Prior Art
Enriched uranium is used as fuel in light water-type nuclear reactors. Since the uranium obtained from the enrichment of uranium is generally in the form of UF.sub.6, it must be converted into UO.sub.2 before it may be used in the reactor. Among the many known methods available for converting UF.sub.6 into UO.sub.2, the ammonium diuranate (referred to hereinafter as ADU for short) method and the gas reaction method have won general recognition.
A typical version of the ADU method is disclosed in Japanese Patent Publication No. 7368/46, published on Feb. 24, 1971. The procedure involved may be roughly described as follows. UF.sub.6, on being brought into contact with an aqueous aluminum nitrate [Al(NO.sub.3).sub.3 ] solution, is converted into uranyl nitrate [UO.sub.2 (NO.sub.3).sub.2 ] in accordance with the following reaction: EQU UF.sub.6 +2H.sub.2 O+2Al(NO.sub.3).sub.3 .fwdarw.UO.sub.2 (NO.sub.3).sub.2 +2AlF.sub.3 +4HNO.sub.3
The resultant solution is subjected to solvent extraction with tributyl phosphate (referred to hereinafter as TBP for short) and is thereby separated into an aqueous solution containing UO.sub.2 (NO.sub.3).sub.2 and a raffinate containing nitric acid [HNO.sub.3 ] and Al(NO.sub.3).sub.3. The extract containing UO.sub.2 (NO.sub.3).sub.2 is then contacted with ammonia gas and the UO.sub.2 (NO.sub.3).sub.2 is thereby converted into ADU, which is precipitated. This ADU is readily separated by filtration, then roasted and subsequently reduced with hydrogen to produce UO.sub.2 powder.
The raffinate mentioned above contains AlF.sub.3, HNO.sub.3 and Al(NO.sub.3).sub.3. When such solution is evaporated to dryness, a vapor containing nitrogen oxide is driven off and the residue is a mixed powder of AlF.sub.3 and Al.sub.2 O.sub.3. Nitric acid recovered by condensation of the vapor may be recycled for the preparation of an aqueous aluminum nitrate solution, but the evaporation process consumes a large amount of heat and the solid state mixed powder of AlF.sub.3 and Al.sub.2 O.sub.3 produced by such process is not particularly valuable as a commercial product. Moreover, the traces of uranium contained in the raffinate are trapped in a concentrated state in the mixed powder making it imperative for the mixed powder to be kept in strict, prolonged confinement as a radioactive waste. Such disadvantages have been borne inevitably by this prior method.
In the gas reaction method, UO.sub.2 powder is obtained by hydrolyzing UF.sub.6 in a gaseous state by direct contact with hot steam and then subjecting the resultant UO.sub.2 F.sub.2 to treatment with hydrogen and hot steam. The steam which is discharged from this process contains hydrogen fluoride which may generally be condensed for recovery of hydrofluoric acid. The hydrofluoric acid thus recovered contains a trace of uranium. Although refined hydrofluoric acid which contains substantially no uranium may be obtained by subjecting the hydrofluoric acid to further distillation, there is little commercial demand for even the refined hydrofluoric acid. Under these circumstances, painstaking efforts are being made by those working in this art in search of some outlet or other use for the hydrofluoric acid so produced.